Process philosophy was an early twentieth century system of thought that was strongly influenced by Albert Einstein’s theory of relativity and other scientific ideas, such as the wave theory of light and sub-atomic physics. The fundamental metaphysical premise of process philosophy is that the basic unit of existence is not a stable thing, such as an atom, but events, or change over time. The two most prominent process philosophers were Alfred North Whitehead (1861–1947) and Charles Hartshorne (1897–2000).